One type of gas discharge laser used in photolithography is known as an excimer laser. An excimer laser typically uses a combination of a noble gas, such as argon, krypton, or xenon, and a reactive gas such as fluorine or chlorine. The excimer laser derives its name from the fact that under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer (or in the case of noble gas halides, an exciplex) is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range.
Excimer lasers are widely used in high-resolution photolithography machines, and are thus one of the critical technologies required for microelectronic chip manufacturing. Current state-of-the-art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with nominal wavelengths of 248 and 193 nanometers respectively.
While excimer lasers may be built with a single chamber light source, the conflicting design demands for more power and reduced spectral bandwidth have meant a compromise in performance in such single chamber designs. One way of avoiding this design compromise and improving performance is by utilizing two chambers. This allows for separation of the functions of spectral bandwidth and pulse energy generation; each chamber is optimized for one of the two performance parameters.
Such dual-gas-discharge-chamber excimer lasers are often called Master Oscillator-Power Amplifier, or “MOPA,” lasers. In addition to improving the spectral bandwidth and pulse energy, the efficiency of the dual chamber architecture can enable the consumable modules in MOPA lasers to reach longer operational lifetimes than their counterpart modules in single chamber light sources.
In each chamber, as the light source discharges energy across its electrodes to produce light, the halogen gas, fluorine in the case of ArF or KrF lasers, is depleted. This causes a decrease in the laser efficiency which is seen, for example, as an increase in discharge voltage required to create a given desired pulse energy. Since the discharge voltage has an upper limit determined by physical constraints of the hardware, steps must be taken to replenish the lost fluorine so that the voltage remains below this limit and the laser continues to function properly.
One way to do this is with a full replenishment of the gas in the chambers, called a refill, where all of the gas is replaced while the laser is not firing to return the gas content in the chamber to the desired mix, concentration and pressure. However, refills are extremely disruptive as the laser be shut off during the refill process, and thus the lithographic exposure of chips must also be paused in a controlled manner at the same time and then restarted when the laser is again operational to avoid improper processing of the chips. For this reason, it is typical to refill both chambers at once to save time, although this is not necessary.
The need for a refill can depend on several complex and often unpredictable variables, including the light source firing pattern and energy, the age of the light source modules, and others that will be familiar to those of skill in the art. For this reason, refills are typically done on a regular schedule, which ensures that the light source operation will never suffer unanticipated interruption due to the light source reaching its operational limit. Such a regular schedule generally yields very conservative upper limits on the time between refills, such that some users of the light source operating at low pulse usages might be able to wait for a much longer period of time between refills than is provided by the simple schedule.
Given the demands of increased throughput and light source availability, efforts have been made to minimize light source stoppage for refills. One way of doing this is by performing a partial replenishment of the gas in the chambers, known as an inject, rather than a full refill. As long as the laser is able to continue to operate within certain parameters, it is not necessary to shut the laser down for the inject, and thus processing of chips may continue during the inject process. However, the performance of the laser still tends to change over time in such a way that injects become inadequate to compensate, and so refills are still performed at regular intervals.
In a refill operation, the remaining gas in the laser chambers is evacuated, and then new gas is introduced into the chambers in an amount intended to arrive at a particular pressure and concentration of fluorine. The pressure and concentration of gas in the laser chambers at the end of a refill is typically determined by the specific type and model of laser (and may even be similar for all dual chamber lasers), and cannot take into account the particular characteristics of a specific laser, such as its age.
Accordingly, a refill may be followed by gas optimization, which is intended to provide the best gas conditions for initial operation of the specific laser. Optimizing the gas allows the specific laser to begin operation at its most efficient point, allowing for longer operation before another refill is required.
Optimization is not always done, in part due to the time required. A refill and optimization typically takes about an hour or more, again with the laser having stopped operation. Rather, whether optimization is done is typically determined by various criteria, such as whether there have been major laser module replacements, or whether there is an observed degradation in the performance of the laser.
To optimize the gas, an engineer test-fires the laser to determine its operating parameters, in particular the discharge voltage and the output energy. If the laser is not operating within the desired parameters, the engineer adjusts the gas in the chambers, and another test-fire done. This is repeated until the desired operating parameters are obtained.
There are some issues inherent in performing gas optimization. The optimization process is typically one of trial and error, so that even an experienced engineer will have some difficulty in obtaining the optimal gas state. This also means that optimization is not easily repeatable; different engineers may produce different optimizations of the same laser, and even a single engineer may not be able to replicate an earlier result. Finally, if errors are made, it may be necessary to repeat the optimization process, resulting in additional downtime of the laser.
A more accurate method of optimizing the gas would mitigate or eliminate many or all of these issues, and allow the laser to operate for a longer period of time before another refill must be performed. Further, a good optimization provides a better basis on which to base the calculation of subsequent injects to the laser chambers. It is thus desirable that optimization be performed in a fashion that results in the most efficient gas state for the particular laser used. The described gas optimization may be particularly effective with an automatic refill process that provides a highly accurate fluorine concentration.